In a broad sense, a heat exchanger is a device used to bring the temperatures of two fluids closer together. The fluids can be gases or liquids, or mixtures of gases and liquids. In most cases, the sole purpose of the heat exchanger is to heat or cool only one of the fluids to within a specific range of temperatures, and the range of temperatures of other fluid is immaterial. In some cases, it is desired to maintain both fluids within specific respective ranges of temperatures. There are two common classifications of heat exchangers: recuperative and regenerative. In the recuperative heat exchanger, the two fluids are isolated from one another in separate confinements by fluidtight thermally conductive walls. In the regenerative heat exchanger, the fluids alternately occupy the same confinement and are isolated from one another by valves which allow each fluid to alternately pass through
In a recuperative heat exchanger, the total heat transfer between the two fluids is a function of and varies because of many factors including the heat transfer between each fluid and the separating walls, which varies typically because of fouling and scaling, the thermal conductivity of the separating walls, and the temperature difference between the fluids. The fluids are generally moved relative to the separating walls which increases the effective output of the heat exchanger. Each fluid is further operating generally within a design range of temperatures, pressure and flow rates; and there are temperature, pressure and flow rate differentials between the fluids.
The regenerative heat exchanger provides that the two fluids can alternately be circulated over a common heat transfer medium. Thus the first fluid (assuming the first fluid is hot and is to be cooled) is passed over the medium to heat the medium and this first fluid is thereby cooled; whereupon the second fluid is then passed over the heated medium to cool the medium, while the second fluid is thereby heated.
Many problems arise for heat exchanger designs capable of operating at temperatures up to 2500.degree. C. and pressure differentials up to several atmospheres. One such problem relates to the materials needed for holding the fluids and/or for moving the fluids about or through the heat exchanger. For instance, most structural steels melt at temperatures in the range of 1350.degree.-1600.degree. C., so that more costly metallic material or high temperature ceramics must be used. Further, when high temperatures and large pressure differentials are involved, the heat exchanger structures become heavier, and the thicker separating walls of a recuperative heat exchanger reduce in direct proportion the effective heat transfer between the fluids. This creates a greater temperature differential as between the two fluids, and requires even larger heat exchanger constructions. The operating temperatures of pumps are frequently held below 400.degree.-600.degree. C., so that few mechanical pump designs are available for circulating the fluids, particularly for pumps of large flow and/or pressure requirements.
The cyclical heating and cooling of the components of the regenerative heat exchanger, especially at very high temperatures, greatly shortens the expected operating life of the unit. Further, the cyclical operation requires that at least two such heat exchangers be arranged in parallel flow circuits with the two fluids (if there is to be continuous flow of either fluid through the heat exchanger system), and the valving is used for directing the first and second fluids alternately through the separate heat exchangers. Moreover, ineffective heat transfer between the fluids and heat transfer medium, and the limited capacity of the heat transfer medium to give up or absorb heat during any single cycle keep the temperature differential between the fluids quite high. The overall effectiveness of this type heat exchanger thereby is quite low.